The present invention generally relates to ductile iron alloys. More particularly, this invention relates to modifying a ductile iron alloy to exhibit desirable properties for turbine compressor case components that must operate at temperatures exceeding the capability of conventional ductile iron alloys.
Various alloys have been considered and used for compressor discharge cases and compressor case and other high-temperature components of industrial gas turbines. Compressor discharge cases are generally located immediately downstream from the compressor of a gas turbine, while compressor cases are still farther downstream and connect compressor discharge cases with the first stage of the turbine section. Because of the high pressures and elevated temperatures sustained between the compressor and turbine sections, alloys suitable for the compressor discharge cases and compressor cases (for convenience, referred to herein simply as compressor cases) require good creep, rupture, tensile, and low cycle fatigue (LCF) properties.
Ductile iron (cast nodular iron) alloys have been developed for various structural applications within turbomachinery and elsewhere due to their strength, toughness, and machinability. As a particular example, the ferritic ductile alloy ASTM A395/A395M-99 has found use as the alloy for pressure-containing structural components used at elevated temperatures, including compressor cases for industrial gas turbines. The ASTM A395/A395M-99 alloy is specified as having a composition of, by weight, at least 3.0% carbon, up to about 2.5% silicon, and up to 0.08% phosphorous, the balance iron and incidental impurities. The ASTM A395/A395M-99 alloy is the current material used in the manufacture of compressor cases for B, F, and E-class technology gas turbines produced by the General Electric Company, such as the MS6001B, MS7001FA, MS7001FB, and MS9001E gas turbine models. Based on the ASTM specification, compressor cases cast from the A395/A395M-99 alloy should be capable of withstanding operating temperatures of up to about 650° F. (about 345° C.). However, as gas turbines are upgraded to promote their performance and efficiency, so do the temperatures and loads that compressor cases must sustain. With such upgrades, additional temperature and stress capability are required as a result of increased pressure ratios and firing temperatures.
The alloying of ductile irons to contain greater amounts of silicon, e.g., about 4 to 6 weight percent, alone or combined with up to about 2 weight percent molybdenum, is known for obtaining higher strengths at high operating temperatures. However, it has been reported that these alloys can exhibit reduced ductility at ambient temperatures, reduced castability, and reduced machinability.